1. Field of the Invention
The present invention relates to an image-sensing apparatus such as a digital video camera.
2. Description of Related Art
There has been proposed resolution enhancement processing—processing whereby one high-resolution image is generated from a plurality of low-resolution images. Such resolution enhancement processing is also called super-resolution processing. The ratio of the number of pixels of the high-resolution image to the number of pixels of the low-resolution image represents the factor of enlargement at which the number of pixels is increased by super-resolution processing. In principle, the factor of enlargement can be made as high as one desires by appropriately setting the number of low-resolution images used for generation of the high-resolution image.
Inconveniently, however, an increase in the number of low-resolution images used for generation of the high-resolution image results in an increase in the calculation burden for super-resolution processing and an increase in the circuit scale needed for the calculation. Moreover, in practice, once the factor of enlargement has reached a certain factor, increasing it further brings hardly any further enhancement in the practical resolution (the maximum spatial frequency expressible in an image). Accordingly, an upper limit is prescribed on the factor of enlargement by which the number of pixels is increased by a resolution enhancement processing portion (super-resolution processing portion) incorporated in an image-sensing apparatus.
The resolution enhancement processing portion can perform super-resolution processing with the factor of enlargement set at any factor equal to or less than the upper-limit factor. The resolution-enhancing effect of the super-resolution processing, however, is maximized when the factor of enlargement at which the number of pixels is increased by super-resolution processing is as high as the upper-limit factor (e.g., because, in a case where the upper-limit factor is 4, setting the actual factor of enlargement at 1.01 does not bring hardly any enhancement in resolution). Accordingly, when super-resolution processing is performed, it is preferable that the ratio of the number of pixels of the high-resolution image to that of the low-resolution image be made equal to the upper-limit factor.
On the other hand, there has been disclosed a method for detecting a specified subject of interest by image processing with a view to cutting out, from the shot image, an image with a composition fit for the subject of interest by clipping processing.
Using this method, it is possible to present the user with a clipped image in which the subject of interest appears large. Inconveniently, however, the clipping processing involved degrades image quality. For example, in a case where the shot image has a number of pixels of 1280 horizontally by 720 vertically (expressed as 1280×720) and from it a clipped image with a number of pixels of (640×360) is cut out, the image presented to the user comes to have ¼ the number of pixels of the shot image.
Such degradation in image quality resulting from clipping processing may be compensated for by super-resolution processing. Inconveniently, however, since, as described above, there is an upper limit on the factor of enlargement at which the number of pixels is increased by super-resolution processing, and in addition there is an optimum value for the factor of enlargement at which the effect of resolution enhancement is maximized, if clipping processing is performed with no consideration given to the number of pixels of the clipped image, it is not possible to present the user with an image with satisfactory quality.